In the gooseneck microphone, a microphone unit is provided on the distal end side of a flexible support pipe including a flexible shaft. The microphone of this type has been used favorably for conferences and the like because the microphone unit can easily be brought close to the mouth of a speaking person.
For the gooseneck microphone, usually, as the microphone unit, a condenser microphone unit has been used. The condenser microphone unit has an impedance converter consisting of a field effect transistor (FET) because the impedance of an electrostatic acousto-electric converter in which a diaphragm and a backplate are arranged opposedly is high.
In recent years, cellular phones have been used everywhere, and are sometimes used even in a conference room. Although not recognized too much, a cellular phone radiates considerably strong electromagnetic waves (producing, in the range of about several centimeters to several tens of centimeters, a field intensity equivalent to several tens of thousands of times the field intensity produced in a city by a commercial power source).
Therefore, if a cellular phone is used near a condenser microphone unit, a high-frequency current caused by the electromagnetic waves radiated from the cellular phone sometimes intrude into the unit. In this case, the high-frequency current is detected by the impedance converter in the unit, and a noise signal caused thereby is superposed on a sound signal, and is delivered from the microphone.
Accordingly, in the invention described in Japanese Patent Application Publication No. 2008-153815, a unit casing of the condenser microphone unit is electrically connected to a support pipe (both made of a metal) to provide grounding, and a shield cup is provided in the unit casing. The microphone unit of this invention is configured so that a microphone cable is brought into the shield cup, the shield cover of the microphone cable is electrically connected to the shield cup, and the impedance converter is housed in the shield cup.
According to this configuration, the unit casing is grounded via the support pipe, and the unit casing is electrically connected to the shield cover of the microphone cable, so that the occurrence of noise caused by the extraneous electromagnetic waves radiated from the cellular phone or the like can be prevented.
However, as the noise generated from the microphone, besides the noise caused by extraneous electromagnetic waves, the noise caused by electric discharge when touched is generated.
That is, when the speaking person touches the microphone unit to bring the microphone unit close to his/her mouth, a spark discharge occurs if the potential difference existing between the speaking person and the microphone unit exceeds the electric breakdown field of air. Thereby, noise is generated from the microphone. Also, even if the spark discharge does not occur, noise is sometimes generated from the microphone in the same manner if a sudden movement of electric charges occurs between the speaking person and the microphone unit.
To prevent the occurrence of noise caused by electric discharge when touched, the unit casing has only to be constructed of a material in which electric charges do not move suddenly, for example, a polymer material (plastic material) having high volume resistivity. In this case, however, the continuity of electrostatic shield is broken in a portion of the polymer material, and the high-frequency current caused by the extraneous electromagnetic waves may intrude into the unit casing from this portion.
Accordingly, an object of the present invention is to provide a gooseneck microphone having a configuration capable of preventing both of the occurrence of noise caused by extraneous electromagnetic waves and the occurrence of noise caused by electric discharge when touched.